1. Field of the Invention
The present invention relates to a method and apparatus for surface-treating any object in the form of lump, sphere, or powder with an atmospheric pressure plasma.
The present invention also relates to a method for surface-treating vulcanized rubber for its joining to other materials.
2. Description of the Prior Art
Among several known methods for surface treatment of objects is one which employs a low-pressure glow plasma. This method permits uniform surface treatment, but in its industrial application it needs a large-scale vacuum unit that produces a vacuum of 10 Torr or below. It also has a high equipment cost and running cost for continuous treatment which is greatly affected by the life of the valve. Another disadvantage is the difficulty in applying to objects made of rubber or plastics which contain a large amount of volatile material. In a reduced pressure, these volatile material evaporate and release themselves from the surface, adversely affecting the desired object, performance, and function of plasma treatment.
There is disclosed in Japanese Patent Laid-Open Nos. 15171/1990, 48626/1990, 241739/1991 and 236475/1991 a method for treating the surface of an object with a glow plasma obtained under atmospheric pressure. This method is suitable for flat objects but is not suitable for objects in the form of lump, sphere, or powder.
Surface treatment of vulcanized rubber has been a common practice where it is necessary to combine vulcanized rubber with other materials (or rubber, metal, or plastics) for the production of composite materials or it is necessary to perform pretreatment on vulcanized rubber for its finish coating. There are several known methods for this purpose.
For example, one of such known methods is used to impart adhesion properties to the surface of vulcanized rubber. It consists of highly oxidizing the surface of vulcanized rubber with a strong acid or strong oxidizing agent, thereby forming minute cracks in the entire surface. However, it suffers from drawbacks involved in using a strong acid or oxidizing agent which needs great handling precaution and seriously damages the properties of vulcanized rubber. In addition, the surface treatment by this method does not provide a sufficient adhesion strength.
There are other methods for the surface treatment of vulcanized rubber. For example, one method consists of treating vulcanized rubber with chlorine gas, and another method consists of treating vulcanized rubber with a pseudohalide compound (See Japanese Patent Publication No. 36910/1977.) These methods are designed to attack the double bonds in rubber, thereby forming Cl-groups which promote adhesion. If these methods are applied to vulcanized rubber to be combined with other materials (such as metal and resin) for the production of anti-vibration rubber, they bring about resinification of treated surface which degrades adhesion properties and heat resistance. And they bring about the yellowing of the treated surface which are serious in the finish coating of golf balls made of balata (transpolyisoprene). They deteriorate the appearance of golf balls. In addition, chlorine gas and pseudohalogen compound are dangerous to the environment.
There is another method for surface treatment. It is a low-pressure glow plasma treating method. According to this method, the surface of vulcanized rubber is treated with O.sub.2 or a mixture of O.sub.2 and CF.sub.4 for etching oxidation and activation. This method permits uniform surface treatment, but in its industrial application it needs a large-scale vacuum unit that produces a vacuum of 10 Torr or below. It also needs a high equipment cost and running cost for continuous treatment. In addition, treatment in a reduced pressure causes the vulcanized rubber to give off oil and water which interfere with the desired performance and function.